Hot-wire anemometry provides a technique whereby a variety of conditions related to the heat absorptive characteristics of a surrounding medium may be studied and interpolated through the monitoring of electrical power supplied to one or more resistive sensors that are placed in the medium under study. Such sensors typically include temperature sensitive hot-wire, hot-film or thermistor elements with known temperature/resistance characteristics which are exposed to the medium and which are used to track and sense the rate of heat transfer by the medium. In combination therewith, balanced resistive bridge circuitry is commonly applied for maintaining a balanced relative condition between the sensors such that, upon detecting a temperature difference therebetween, reflected in a change of resistance and voltage, power is applied to at least one of the sensors to re-establish equilibrium. The application of the power may then be monitored and used in the determination of a parameter of interest. Examples of devices of this type can be seen upon directing attention to U.S. Pat. Nos. 3,603,147, and 4,159,638 and an article by Sasayama et al, entitled "Engine Control System Using a Hot-Wire Airflow Sensor", Hitachi Review, 31:61-66 (1982).
While various control techniques can be employed relative to the resistive sensors, the present invention controls sensor temperature by forcing a certain known resistance. Upon placing the sensor within a flowing medium, changes in flow velocity thus induce unique monitorable changes in the convective power dissipation from the sensor. Such power changes are related to the current supplied in accordance with the following equations: ##EQU1## where the variable of interest V is the velocity of the medium and .DELTA.T is the temperature of the sensor above the fluid medium.
Because changes in the fluid medium's ambient temperature can affect the sensed output, it is necessary to monitor the changing temperature in the fluid medium and make corresponding changes in the temperature of the temperature compensating sensor in order to maintain a constant overheat condition (.DELTA.T) or relation between the sensed fluid temperature and the velocity detecting sensor. Upon eliminating of the effects of changing fluid temperature, velocity is the only variable and which as mentioned is determined by measuring the power delivered to the velocity sensor. This constant overheat condition is achieved for bridge-type anemometers by referencing the temperature of a velocity determining sensor to a set value (determined by a series coupled resistance) above the temperature of a temperature compensating sensor that tracks the fluid temperature. This type of anemometer is particularly known as a constant overheat anemometer and when implemented in a Wheatstone type bridge anemometer, a balanced condition is maintained between a velocity sensing portion and a temperature sensing portion.
While such constant overheat anemometers have proved extremely useful in a variety of applications, a problem which affects device accuracy is that the conductors over which comparative balanced conditions are sensed are the same conductors as over which the current is supplied for changing the sensor temperature and thus resistance and voltage. Because, too, the power dissipation in the velocity sensor portion is greater than that in the temperature compensation sensor portion, a disparity occurs in the resistances of the individual sensor containing conductor paths which is reflected back in erroneous power consumption data and consequently flow data.
Accordingly, it is a primary object of the present invention to overcome the foregoing problem through the use of control circuitry wherein the voltage sensing conductors are isolated from the current supply conductors. In this regard, too, the fluid temperature sensing function is made to be no longer dependent upon the absolute resistance of the temperature compensation sensor, but instead, now depends upon a current ratio between the temperature compensation leg relative to the velocity sensor leg.
Also provided in the present invention is cross-coupling circuitry in the velocity sensing portion for rejecting any common mode signals present therein. The change in resistance of the lead wire is thus rejected. A further benefit from the combination of the above features in the present circuitry is the ability to employ matched sensors. It is no longer necessary to calibrate the otherwise unmatched sensors relative to one another, since accurate results can now be achieved directly.
The foregoing objects, advantages and distinctions of the present invention over the prior art as well as the construction thereof will, however, become more apparent upon directing attention to the following description with respect to the appended drawings. Before referring thereto, though, it is to be recognized that the description is made with respect to the presently preferred embodiments only and that various modifications may be made thereto without departing from the basic invention.